1. Field of the Invention
The present invention relates to a stepping motor drive device, and in particular relates to techniques of driving a stepping motor with low noise and low vibration.
2. Related Art
In recent years, stepping motors are used in photographing electronics devices, such as a DSC (Digital Still Camera) and a DVC (Digital Video Camera), as optical system actuators for adjustment of aperture, focus, zoom, and the like.
Stepping motors, especially those used for photographing electronics devices, are required to operate with low noise and low vibration. This is because sounds generated by a stepping motor are caught by a built-in microphone of a photographing electronics device and recorded as noise, and vibrations generated by a stepping motor cause unsteadiness of a photographing electronics device and result in a drop in picture quality.
In response to this demand, a technique of driving a stepping motor with low noise and low vibration is disclosed, for instance, by Unexamined Japanese Patent Application Publication No. H06-343295.
FIG. 23 is a drive device disclosed by this publication. The following description focuses only on components that are necessary for explaining the principle of the device.
In FIG. 23, reference numeral 20 denotes a stepping motor that is driven by the drive device. The stepping motor 20 includes a rotor 45, a first coil 19a, and a second coil 19b. 
An up-down counter 43a counts up or down clock signal CLKP in accordance with up-down signal DA, and outputs 4-bit signal DA1-DA4 showing the count to a D/A converter 44a. 
The D/A converter 44a outputs voltage signal VCA corresponding to 4-bit signal DA1-DA4. Voltage signal VCA is a staircase waveform that rises and falls stepwise. A rapidity of change in level of voltage signal VCA can be varied depending on a pulse frequency of clock signal CLKP and a step height of voltage signal VCA. When the pulse frequency of clock signal CLKP is higher or the step height of voltage signal VCA is greater, the rapidity of change in level of voltage signal VCA is greater. Also, by stopping clock signal CLKP, voltage signal VCA can be held at a fixed level.
A voltage drive circuit 39a amplifies voltage signal VCA by a non inverting power amplifier 41a, and also amplifies voltage signal VCA by an inverting power amplifier 42a. Thus, the voltage drive circuit 39a drives the first coil 19a which is connected between output terminals of the power amplifiers 41a and 42a, through the application of a voltage.
The same components as described above are provided for the second coil 19b too, to drive the second coil 19b through the application of a voltage based on a staircase waveform.
According to this construction, a voltage which rises stepwise at the beginning of energization and falls stepwise at the end of energization is applied to a coil. Hence vibration and noise caused by abrupt torque fluctuations can be reduced when compared with the case where a voltage which rises to a highest level in one stroke at the beginning of energization and falls in one stroke at the end of energization is applied to a coil.
Note here that though the above drive device drives each coil through the application of a voltage, the same noise and vibration suppression effect can still be achieved even when each coil is driven through the application of a current based on a staircase waveform.
The effect of this conventional drive device, however, is not sufficient to completely eliminate vibration and noise, since some vibration and noise still remain due to torque fluctuations which occur in each step-up and step-down of the staircase waveform. This is particularly problematic when stepping motors are used in photographing electronics devices. Hence a need exists for further reduction of noise and vibration in stepping motor operations.